This application is based on Japanese Patent Application No. 2000-181264 filed Jun. 16, 2000, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a vibration damping rubber member capable of exhibiting excellent durability and a method of producing the same, and more particularly to a vibration damping rubber member which contributes to reduction of the weight of automotive vehicles when the vibration damping rubber member is used on the automotive vehicles, and a method suitable for producing such a vibration damping rubber member.
2. Discussion of Related Art
As well known, a vibration damping rubber member interposed between two members in a vibration or shock transmitting system so as to exhibit vibration damping or cushioning characteristics has been widely used in various fields. For instance, the vibration damping rubber member is used on automotive vehicles, as engine mounts, body mounts, cab mounts, member mounts, strut mounts, strut bar cushions, suspension bushings, and so on.
The automotive vehicles on which such a vibration damping rubber member is installed are generally required to have a reduced weight in order to improve fuel economy and to achieve high performance. The vibration damping rubber member used on the automotive vehicles is accordingly required to have a reduced weight.
As one technique for reducing the weight of the vibration damping rubber member interposed between two members in the vibration or shock transmitting system, two supporting or mounting members for supporting the vibration damping rubber member which are attached to the respective two members in the vibration system are formed of an aluminum material or a resin material in place of a conventionally used metallic material. As another technique, the vibration damping rubber member is designed to have a reduced size. In the vibration damping rubber member which is made compact to reduce its weight, the volume (amount) of the rubber is inevitably smaller than that in a conventional vibration damping rubber member. Accordingly, the material used for the vibration damping rubber member whose size is reduced needs to exhibit a considerably high degree of durability for withstanding a vibrational load as large as that applied to the conventional vibration damping rubber member.
The present invention is made in view of the background art described above. It is a first object of this invention to provide a vibration damping rubber member which is capable of exhibiting excellent durability without suffering from deterioration of its spring stiffness.
It is a second object of the invention to provide a method suitable for producing such a vibration damping rubber member.
The first object indicated above may be achieved according to a first aspect of the present invention, which provides a vibration damping rubber member having excellent durability, wherein an improvement comprises: the vibration damping rubber member being obtained by vulcanizing a composition in which an unvulcanized natural rubber material and an unvulcanized acrylic rubber material are evenly mixed together in a proportion of 90/10xcx9c60/40 by weight; and fine particles of a vulcanized acrylic rubber having a size of 1xcx9c100 xcexcm and obtained by vulcanization of the unvulcanized acrylic rubber material being dispersed in a matrix phase of a vulcanized natural rubber obtained by vulcanization of the unvulcanized natural rubber material.
The vibration damping rubber member constructed according to the first aspect of the present invention is characterized in that a suitable amount of the fine particles of the vulcanized acrylic rubber obtained by vulcanization of the unvulcanized acrylic rubber material are dispersed in a matrix phase of the vulcanized natural rubber obtained by vulcanization of the natural rubber material. In the thus formed vibration damping rubber member, the acrylic rubber dispersed in the form of the fine particles in the matrix phase of the natural rubber permits even distribution of the load applied to the vibration damping member. Accordingly, the present vibration damping rubber member exhibits significantly improved durability without suffering from deterioration of its spring stiffness (dynamic/static ratio of spring constant).
The vibration damping rubber member of the present invention is obtained by vulcanization of the composition in which the unvulcanized natural rubber material and the unvulcanized acrylic rubber material are mixed in a proportion of 90/10xcx9c60/40 by weight. In the formed vibration damping rubber member, the fine particles of the vulcanized acrylic rubber having a size of 0.1xcx9c100 xcexcm and obtained by vulcanization of the unvulcanized acrylic rubber material are dispersed in the matrix phase of the vulcanized natural rubber obtained by vulcanization of the unvulcanized natural rubber material. According to this structure, the vibration damping rubber member effectively exhibits improved durability while assuring physical properties required by the vibration damping rubber member.
The second object indicated above may be achieved according to a second aspect of the present invention, which provides a method of producing a vibration damping rubber member, the method comprising the steps of: preparing an unvulcanized rubber composition by evenly mixing together an unvulcanized natural rubber material and an unvulcanized acrylic rubber material in a proportion of 90/10xcx9c60/40 by weight, and adding a vulcanizing agent for vulcanizing the unvulcanized natural rubber material and a vulcanizing agent for vulcanizing the unvulcanized acrylic rubber material; forming a thus obtained mixture into a desired shape; and vulcanizing the formed mixture for obtaining the vibration damping rubber member in which fine particles of a vulcanized acrylic rubber having a size of 0.1xcx9c100 xcexcm and obtained by vulcanization of the unvulcanized acrylic rubber material are dispersed in a matrix phase of a vulcanized natural rubber obtained by vulcanization of the unvulcanized natural rubber material.
The present method of producing a vibration damping rubber member permits the vulcanized acrylic rubber obtained by vulcanization of the unvulcanized acrylic rubber material to be evenly dispersed in the form of the fine particles in the matrix phase of the vulcanized natural rubber obtained vulcanization of the unvulcanized natural rubber. Accordingly, the vibration damping rubber member having a high degree of durability can be advantageously produced according to the present invention.
The vibration damping rubber member according to the present invention constructed as described above is constituted by an intimate mixture of a vulcanized natural rubber and a vulcanized acrylic rubber, which is obtained by vulcanization of a natural rubber material and an acrylic rubber material known in the art. The present vibration damping rubber member is characterized in that the fine particles of the vulcanized acrylic rubber having a size of 0.1xcx9c100 xcexcm are dispersed in the matrix phase of the vulcanized natural rubber.
The unvulcanized natural rubber material and the unvulcanized acrylic rubber material used for forming the present vibration damping rubber member are both polymers which are mutually incompatible. In the vibration damping rubber member obtained by vulcanization of the two unvulcanized rubber materials which ate mixed together in a predetermined proportion, the vulcanized natural rubber and the vulcanized acrylic rubber are not mutually compatible. In the present vibration damping rubber member, the vulcanized acrylic rubber is dispersed, in the matrix phase of the vulcanized natural rubber, in the form of fine particles having a size of 0.1xcx9c100 xcexcm. The fine particles of the vulcanized acrylic rubber dispersed in the matrix phase of the vulcanized natural rubber are effective to distribute and reduce the load applied to the vibration damping member, for thereby considerably improving the durability of the vibration damping member. It is preferable that the fine particles of the vulcanized acrylic rubber have a size of not larger than 10 xcexcm for further improved durability of the vibration damping member. The size of the fine particles of the vulcanized acrylic rubber exceeding the upper limit of 100 xcexcm adversely influences the physical properties such as breaking elongation of the vibration damping member. The particle size of the vulcanized acrylic rubber may be measured by various known methods. For instance, the particles of the vulcanized acrylic rubber are observed by a scanning electron microscope (SEM) or a scanning probe microscope (SPM), to measure the sizes of the particles.
For permitting the vibration damping rubber member formed as the end product to exhibit the desired operating characteristics or physical properties, the ratio of the weight of the unvulcanized natural rubber material to that of the unvulcanized acrylic rubber material is selected within a range between 90/10 and 60/40. If the amount of the unvulcanized acrylic rubber material is excessively smaller than that of the unvulcanized natural rubber material, the produced vibration damping rubber member does not enjoy the effect to be obtained by addition of the unvulcanized acrylic rubber material. If the amount of the unvulcanized acrylic rubber material is excessively larger than that of the unvulcanized natural rubber material, on the other hand, the physical properties such as permanent compressive strain of the vibration damping rubber member may be deteriorated. Where the amount of the unvulcanized acrylic rubber material is extremely larger than that of the unvulcanized natural rubber material, the vulcanized acrylic rubber, rather than the vulcanized natural rubber, may undesirably be a matrix phase in the produced vibration damping rubber member. In this case, the vibration damping rubber member may not have the desired structure described above and the desired properties required by the vibration damping rubber member.
In the present invention, the natural rubber material and the acrylic rubber material need to be mixed together in an unvulcanized state. If a vulcanized acrylic rubber instead of the unvulcanized acrylic rubber is mixed with the unvulcanized natural rubber, it is speculated that the vibration damping rubber member to be produced by vulcanization of the thus obtained mixture does not have satisfactory physical properties such as breaking elongation required by the vibration damping rubber member, due to a considerably small degree of interaction at an interface between the natural rubber and the acrylic rubber. In other words, the produced vibration damping rubber member is not constituted by an intimate mixture of the vulcanized natural rubber and the vulcanized acrylic rubber.
The natural rubber material and the acrylic rubber material used for producing the present vibration damping rubber member are suitably selected from among those known in the art. For instance, the acrylic rubber material may be selected as needed, depending upon the desired characteristics required by the vibration damping member, from among any known synthetic rubber materials whose major component is alkyl acrylate. In particular, it is preferable to use acrylic rubber materials which can be vulcanized with a polyamine-based vulcanizing agent which will be described. For instance, it is preferable to use a copolymer (ACM) of alkyl acrylate and 2-chloroethyl vinyl ether, a copolymer (ANM) of alkyl acrylate and acrylonitrile, and a copolymer (VAMAC-G) of methyl acrylate and ethylene.
In producing the present vibration damping rubber member having the intended structure by using the unvulcanized natural rubber material and the unvulcanized acrylic rubber material described above, suitable amounts of known vulcanizing agents suitable for vulcanizing the respective rubber materials are added to the mixture of the unvulcanized rubber materials prior to its vulcanization. As the vulcanizing agent for vulcanizing the unvulcanized natural rubber material, a sulfur-based vulcanizing agent such as sulfur is generally used, so that the unvulcanized natural rubber material is vulcanized by a known sulfur-based vulcanizing system. The vulcanizing agent for vulcanizing the unvulcanized acrylic rubber material is suitably selected from among any known vulcanizing agents depending upon the kinds of the acrylic rubber material to be used. For instance, the polyamine-based vulcanizing agent such as hexamethylene diamine carbamate or triethylene tetramine is used.
In the present invention, at least one selected vulcanization promoting agent and/or aid may be used in combination with the respective vulcanizing agents for vulcanizing the two unvulcanized rubber materials. The appropriate vulcanization promoting agent(s) and/or aid(s) is/are selected depending upon the specific unvulcanized rubber material (the unvulcanized natural rubber material or the unvulcanized acrylic rubber material) and the specific vulcanizing agent. The selected at least one vulcanization promoting agent and/or aid is mixed and kneaded in a suitable amount with the specific unvulcanized rubber material and the specific vulcanizing agent, so that the unvulcanized rubber material is vulcanized with an improved result.
Typical examples of the vulcanization promoting agent include: sulfenamides such as N-tert-butyl-2-benzothiazolylsulfenamide (BBS), N-cyclohexyl-2-benzothiazolylsulfenamide (CBS), and N-oxydiethylene-2-benzothiazolylsulfenamide (OBS); dithiocarbamates such as zinc dimethyldithiocarbamate (ZnMDC) and zinc diethyldithiocarbamate (ZnEDC); and thiurams such as tetramethyl thiuram disulfide (TMTD), tetraethyl thiuram disulfide (TETD), and tetrabutyl thiuram disulfide (TBTD). As the vulcanization promoting aid, zinc oxide or stearic acid is used, for instance.
Various other suitable known additives may be added to the unvulcanized rubber materials, as needed. Those additives may include: reinforcing agents such as carbon black; anti-aging agents such as wax; and softening agents such as oil. It is noted that these additives should not prevent the produced vibration damping rubber member from exhibiting the desired physical properties and operating characteristics and the amounts of the additives should be determined so as not to deteriorate those physical properties and operating characteristics of the vibration damping rubber member.
In producing the present vibration damping rubber member, various methods known in the art can be employed. In the present invention, the unvulcanized natural rubber material and the unvulcanized acrylic rubber material are mixed in a suitable ratio by weight, i.e., the ratio of the weight of the natural rubber material to that of the acrylic rubber material=90/10xcx9c60/40, and the mixture is introduced into a suitable known kneader or mixer such as a banbury mixer or a mixer of roll type, so that the mixture is evenly kneaded. To this mixture, the vulcanizing agents for vulcanizing the respective unvulcanized rubber materials and the suitably selected additives are added and kneaded, so as to provide an unvulcanized rubber composition in which the unvulcanized acrylic rubber material is evenly dispersed in the form of fine particles having a predetermined size in the unvulcanized natural rubber material. Subsequently, the thus obtained unvulcanized rubber composition is formed into a desired shape by a suitable molding method using a molding die, for instance, at a suitably determined temperature at which the unvulcanized natural rubber material and the unvulcanized acrylic rubber material are concurrently vulcanized. Thus, the intended vibration damping rubber member is produced.
The order of introducing, into the kneader, the unvulcanized natural rubber material, unvulcanized acrylic rubber material, vulcanizing agents for the respective unvulcanized rubber materials, and additives is not limited to that described above. For instance, all components may be simultaneously introduced into the kneader or mixer. Alternatively, all components except the vulcanizing agents are introduced into the kneader or mixer for preliminary kneading, and the vulcanizing agents may be added in a subsequent step of finish kneading. Namely, the components are mixed together such that the unvulcanized natural rubber material and the unvulcanized acrylic rubber material are vulcanized concurrently, or successively after the vulcanizing agents have been added.
The kneading operation is effected at a suitable temperature for a suitable time period. In the present invention, in order to establish the desired state of dispersion of the fine particles of the acrylic rubber material, the kneading time is determined in view of the specific kinds and the amounts of the natural rubber material and the acrylic rubber material, and the operating characteristic of the kneader.
In molding and vulcanizing the unvulcanized rubber composition, the vulcanizing condition such as the temperature, pressure and time is suitably determined depending upon the specific kinds of the unvulcanized natural rubber material, the unvulcanized acrylic rubber material, and the vulcanizing agents, for achieving effective vulcanization of those unvulcanized rubber materials. The molding and vulcanizing operation may be effected according to any known manner such as a press-molding and -vulcanizing process in which the molding and the vulcanization are effected concurrently. The vibration damping rubber member to be produced may be provided with a metallic structure made of a ferrous or aluminum material, which may be bonded to the rubber member during or after the molding and vulcanizing operation. In this respect, it is to be understood that the method of the present invention is applicable to not only a vibration damping rubber member without such a metallic structure, but also a vibration damping rubber member with such a metallic structure bonded thereto. It is also to be understood that the configuration and size of the vibration damping rubber member to be produced by vulcanization of the unvulcanized natural rubber material and the unvulcanized acrylic rubber material are not particularly limited, but may be suitably determined depending upon the desired characteristics and application of the vibration damping rubber member.
The vibration damping rubber member produced as described above is interposed between two members in a vibration or shock transmitting system of an automotive vehicle, so as to function as an engine mount, body mount, cab mount, member mount, strut mount, strut bar cushion, suspension bushing, and so on. Since the present vibration damping rubber member can be made compact owing to the significantly improved durability, the automotive vehicle on which the present vibration damping rubber member is installed has an advantageously reduced weight.